PROJECT SUMMARY Ewing sarcoma is the second most common bone cancer in children, and patients with metastatic disease have less than a 25% chance of survival. It is imperative that we better understand the mechanisms that drive metastasis and find new ways to treat these patients. Crosstalk between tumor cells and the tumor microenvironment (TME) is critical in driving metastasis, leading me to hypothesize that Ewing sarcoma metastasis is dependent on both tumor cells themselves as well as the extracellular matrix (ECM). This hypothesis is informed by previously published data from our lab that revealed a role for Wnt/?-catenin signaling and the Wnt/?-catenin target gene, Tenascin C (TNC), in promotion of a metastatic phenotype. I specifically observed an increase in expression of genes associated with the ECM and involved in mediating tumor: TME interactions post activation of Wnt/?-catenin. In Aim 1.1, I tested the hypothesis that Wnt/?-catenin changes the Ewing sarcoma secretome. Here, I show that we see a change in abundance and identity of ECM proteins that are secreted by Ewing sarcoma cells, specifically I see an increase in TNC and collagen I. I then go on to show that TNC is necessary for Ewing sarcoma invasion and matrix remodeling. In the secretome of Wnt-activated Ewing sarcoma cells, I also observed enrichment for proteins that are direct targets of the TGF? signaling cascade and proteins known to interact with TGF? ligands in the TME. Previous reports show that Ewing sarcoma cells are unresponsive to TGF? ligands. However, investigation into the potential link between TGF? and Wnt/?-catenin in Ewing sarcoma revealed that Wnt-responsive cells are sensitized to TGF? signaling and as a result, able to activate the pathway in vitro. In Aim 1.2, I will determine whether or not activation of Wnt and TGF? signaling promote phenotypic changes in both the tumor cells and the surrounding non-tumor cells by looking at tumor cell invasion, endothelial cell proliferation, and osteoclast activation. Additionally, I will identify the TGF? dependent transcriptome and determine how much of the Wnt dependent transcriptome is dependent on sensitization and subsequent activation of TGF? signaling. Together this work will propose a novel connection between the Wnt/?-catenin and TGF? signaling cascades in promoting a metastatic phenotype in Ewing sarcoma. In Aim 2, I will pursue a post-doctoral fellowship that will allow me to dive deeply into the study of cancer cell cooperation. It is evident that different subpopulations within a tumor cooperate to promote an aggressive phenotype more efficiently than each population would independently. Addressing the complex issues of cellular heterogeneity and cell cooperation in tumor progression requires advanced use of tumor models as well as sophisticated bioinformatics tools that permit identification of clonal subpopulations and the functional consequence of their interactions. By combining skills learned in my PhD thesis and Master?s degree in Bioinformatics, I will be poised to succeed in interdisciplinary research aimed at understanding how clonal cooperation contributes to tumor progression.